1. Field of the invention
The present invention relates to a call signal detecting device and, more particularly, to a call signal detecting device applicable to a facsimile communication system for identifying call signals which come in from a network.
2. Description of the Background
A public switched telephone network uses a call signal to be received by a subscriber which is generally implemented by a 16-hertz high-voltage call signal, for example. A facsimile communication network, on the other hand, uses a facsimile call signal which may comprise a 1,300-hertz unsounding call signal. Preferably, therefore, a facsimile apparatus connected to such networks and furnished with an automatic call incoming function should be constructed to identify both of sounding call incoming and unsounding call incoming and, yet, to clearly discriminate them from off-hooking and subsequent dialing performed at the own station's telephone set.
In light of the above, a prior art call signal detecting device has relied on an unsounding call signal detection circuit, a sounding call signal detection circuit, and an off-hook detection circuit which are independent of each other. The call signal detection circuits are each connected to a subscriber's line ahead of a switching contact which leads to a facsimile body, that is, at the central station side, while the off-hook detection circuit is connected to a lead-in loop leading to a telephone set. The unsounding call signal detector may comprise an amplitude limiting amplifier, a filter, and a detector which are cascaded with a secondary winding of a pickup transformer a primary winding of which is connected to a subscriber's loop via a dc cutoff capacitor. The unsounding call signal detector having such a construction detects the 1,300-hertz frequency signal in terms of an increase in the output level of the detector beyond a predetermined one. The sounding call signal detector, on the other hand, may comprise a relay connected to the subscriber's line via a full-wave rectifier, which is implemented by a diode bridge, and caused to respond to a rise of the output of the rectifier beyond a predetermined level by detecting a 16 hertz ac call signal.
Each of the call signal detectors is provided with a gate at its output side. The gate is disabled by an output of the off-hook detector responsive to off-hooking of the own station's telephone set and, thereby, prevented from malfunctioning in response to dial pulses associated with the own station's telephone set or multi-frequency (MF) signals. Meanwhile, since the detection of such incoming signals is effected in terms of the signal level, outputs of the detectors have to be fed to the facsimile apparatus independently of each other and, therefore, a 3-bit output port is required. In this manner, the prior art device is complicated in construction and cannot meet the increasing demand for a small-size construction.
Further, in the unsounding call signal detector, the unsounding call signal cannot be discriminated from an address signal generated by a push-button telephone set, or an MF signal, unless two discrete detector circuitries are provided at the output side of the amplitude limiting amplifier. Specifically, an output of the amplifier is routed through a band-pass filter to a detector on one hand and through a band-rejection filter to another detector on the other hand. A gate provides AND of the outputs of the two detectors to determine that an unsounding call signal has been detected only when the output of the band-pass filter side is high level and that of the band-rejection filter side is low level. Hence, where the incoming call signals have to be discriminated from MF signals, the construction becomes even more complicated and, especially, the filters need be highly accurate analog filters.